1. Field of the Invention
This invention generally relates to implantable devices, such as an expandable intraluminal prosthesis commonly known as stents. More particularly, this invention relates to a structures and techniques for applying therapeutic substances to an implantable device in association with the implantation procedure.
2. Description of the Related Art
Percutaneous transluminal coronary angioplasty (PTCA) is a procedure for treating heart disease. A catheter assembly having a balloon portion is introduced into the cardiovascular system of a patient via the brachial or femoral artery. The catheter assembly is advanced through the coronary vasculature until the balloon portion is positioned across the occlusive lesion. Once in position across the lesion, the balloon is inflated to a predetermined size to radially compress the atherosclerotic plaque of the lesion against the inner wall of the artery to dilate the lumen. The balloon is then deflated to a smaller profile to allow the catheter to be withdrawn from the patient's vasculature.
A problem associated with the above procedure includes formation of intimal flaps or torn arterial linings which can collapse and occlude the conduit after the balloon is deflated. Moreover, thrombosis and restenosis of the artery may develop over several months after the procedure, which may require another angioplasty procedure or a surgical by-pass operation. To reduce the partial or total occlusion of the artery by the collapse of arterial lining and to reduce the chance of the development of thrombosis and restenosis, an expandable intraluminal prosthesis, an example of which includes a stent, is implanted in the lumen to maintain the vascular patency. A well known procedure for delivering the stent to the diseased site includes crimping a compressed stent about the balloon of the catheter such that when the balloon is inflated, the stent dilates and is disposed within the vasculature. FIG. 1 illustrates an example of the end result, the balloon having been deflated and withdrawn. FIG. 1 shows a stent 10, generally tubular in shape, in its expanded position, functioning to hold open and, if desired, to expand a segment of an anatomical lumen 12. As best shown by FIG. 1, stent 10 prevents torn or injured arterial lining 14 from occluding lumen 12.
In treating the damaged vasculature tissue and to further fight against thrombosis and restenosis, there is a need for administrating therapeutic substances to the treatment site. For example, anticoagulants, antiplatelets and cytostatic agents are commonly used to prevent thrombosis of the coronary lumen, to inhibit development of restenosis, and to reduce post-angioplasty proliferation of the vascular tissue, respectively. To provide an efficacious concentration to the treated site, systemic administration of such medication often produces adverse or toxic side effects for the patient. Local medication delivery is a preferred method of treatment in that smaller total levels of medication are administered in comparison to systemic dosages, but are concentrated at a specific site. Local delivery thus produces fewer side effects and achieves more effective results. One commonly applied technique for the local delivery of the drugs is through the use of medicated stents. Stents that are capable of storing medication and releasing it at the implanted site are well known in the art. A metallic stent coated with a polymeric material which is impregnated with a drug or a combination of drugs is one example. Once the stent is implanted within the lumen, the drug(s) are released from the polymer. U.S. Pat. No. 5,605,696 to Eury et al., U.S. Pat. No. 5,464,650 to Berg et al., and U.S. Pat. No. 5,700,286 to Tartaglia et al. are examples illustrating the use of a polymeric coating for the local delivery of the drug(s).
Sterilization of medicated stents in preparation for stent therapy significantly limits the choice of drugs with which the stent can be medicated. More specifically, stents are sterilized by ethylene oxide (Eto) gas or electron beam radiation. Some therapeutic substances do not tolerate either the Eto or electron beam radiation procedure. Although some therapeutic substances tolerate Eto, Eto is the less preferred method of sterilization for coronary procedures since the procedure leaves an ethylene residue on the stent after sterilization, which can provoke an inflammatory response.
The available choice of therapeutic substances for medicating stents therefore includes substances that are not adversely affected by electron beam radiation. The selections are limited. Accordingly, it is desirable to medicate the stent subsequent to the sterilization procedure.
Medicated stents also inhibit a treating physician's ability to make an ad hoc selection of most suitable therapeutic substance or combination of therapeutic substances, and dosage for a particular patient. A physician cannot custom treat a stent according to a patient's needs, but rather is limited to selections that are already provided by a biomedical supplier. Accordingly, it is desirable to allow a physician to medicate the stent in accordance with the particular needs of a patient.
Stents are medicated by a biomedical supplier well in advance of the stent therapy procedure and supplied to users in sterile packages. The therapeutic substance concentration that is secured by the stent diminishes during storage in sterile packages due to inevitable diffusion of the substance from the stent. The time lapse between treating a stent with a therapeutic substance and implanting the stent may decrease the therapeutic substance's efficacy or require the package to be discarded if extending beyond the package expiration date. Accordingly, it is desirable to medicate a stent immediately prior to the stent therapy.